This is much too large a question for anyone to answer here. I would suggest that you take a look at the wikipedia page on Dark Matter. Then, if you still have questions, I'm sure people will be happy to help.

The evidence is mostly gravitational. The Universe behaves (on the large scale) as though it were being pulled upon with gravitational forces that indicate far more mass than the amount that we can see. We assume that this gravitational force is the result of the presence of mass (like all the gravitational fields we know of), and that this mass is caused by the rpesence of matter, but this matter is not visible to us because it is not radiating light, like the stars. It is matter that we cannot see, therefore "dark matter".

The significant question arises from the evidence that most dark matter is non-baryonic, which means we don't know what it's made of.

The conclusion that most of the DM is non-baryonic is theory dependent, i.e. it is dependent on a Friedmann model of the expansion of the universe in the first three minutes. In the radiation dominated era [itex]R(t) \propto t^{1/2}[/itex].

However the strictly linearly expanding model [itex]R(t) \propto t[/itex] appears to produce just about the right amount of baryonic matter to account for nearly all of DM. A Concordant “Freely Coasting” Cosmology

Interestingly, the baryon entropy ratio required for the right amount of helium corresponds to
[itex]\Omega_b \approx 0.2[/itex]. Here [itex]\Omega_b[/itex] is the ratio of the baryon density to a “density parameter” determined by the Hubble constant: [itex]\Omega_b = \rho_b/\rho_c = 8\pi G \rho_b/3H_o^2[/itex]
[itex]\Omega_b \approx 0.2[/itex] closes dynamic mass estimates of large galaxies and clusters [see eg [20, 21]]. In standard cosmology this closure is sought to be achieved by taking recourse to non-baryonic cold dark matter. Thus in a linearly scaling cosmology, there would be no need of non-baryonic cold dark matter to account for large scale galactic flows.

The conclusion that most of the DM is non-baryonic is theory dependent, i.e. it is dependent on a Friedmann model of the expansion of the universe in the first three minutes. In the radiation dominated era [itex]R(t) \propto t^{1/2}[/itex].

However the strictly linearly expanding model [itex]R(t) \propto t[/itex] appears to produce just about the right amount of baryonic matter to account for nearly all of DM. A Concordant “Freely Coasting” Cosmology

Garth

I was under the impression that, regardless of cosmological models, baryonic dark matter would lead to the wrong kind of structure for galactic and cluster halos, due to EM interactions.

That could well be right, it depends on what form the baryonic DM is in today.

IMBH's, for example, would interact gravitationally and not through EM interactions.

If the DM is baryonic I would expect it to consist of about 50% IMBHs, the remnants of an era of PopIII stars, and ~ 50% WHIM and cold gas.

At high z there would be a longer cosmological age than in the standard model during which the large structure might form.

Garth

My worry was less about large scale structure and more about fitting the results from galactic rotation curves and gravitational lensing. EM interactions provide a way for matter to radiate energy and angular momentum, which should lead to smaller halos.

Also, if IMBHs were so prevalent as to account for 50% of dark matter, shouldn't we be able to see lensing due to some of them?

My worry was less about large scale structure and more about fitting the results from galactic rotation curves and gravitational lensing. EM interactions provide a way for matter to radiate energy and angular momentum, which should lead to smaller halos.

And therefore we can invent non-baryonic DM with just the right properties? I'll believe it when the DM particle is discovered in the laboratory and found to have just those properties!

Also, if IMBHs were so prevalent as to account for 50% of dark matter, shouldn't we be able to see lensing due to some of them?

The required properties depend on exactly what feature is being modelled, the size and density distribution of halos, their 'cuspyness', the galactic rotation curve etc.

The point I was making was that the model fitting uses hypothetical particles, which may or may not actually exist. Until such particles are discovered in the laboratory then such models are just as speculative as alternative theories which modify GR.

There should be less confidence in the standard model and more attention paid to possible alternatives until such laboratory discoveries, confirming the standard model, are made.

The conclusion that most of the DM is non-baryonic is theory dependent, i.e. it is dependent on a Friedmann model of the expansion of the universe in the first three minutes. In the radiation dominated era .

An additional observation in favor of the current estimation of baryonic matter (about 4%) is the ratio of H1 to H2 in the universe, as well as other ratios of nuclides formed right after the big bang.

An additional observation in favor of the current estimation of baryonic matter (about 4%) is the ratio of H1 to H2 in the universe, as well as other ratios of nuclides formed right after the big bang.

By H2 do you mean Deuterium? In which case you are correct; in the linearly expanding model the Deuterium is destroyed in the intial BBN and has to be explained by a spallation process, probably in the shocks associated with supernovae, such as at the supposed demise of PopIII stars.

An additional observation in favor of the current estimation of baryonic matter (about 4%) is the ratio of H1 to H2 in the universe, as well as other ratios of nuclides formed right after the big bang.

Quite right. Astronomical nomenclature can be confusing, but generally...

2H - deuterium
H2 - molecular hydrogen
HII - ionized hydrogen

The zeroth order result from nucleosynthesis that most people quote is the ratio of helium to hydrogen, though this is a less sensitive indicator of cosmology than some of the rarer isotopes that are more difficult to produce (like deuterium).